In industrial transmitter applications, capacitive type sensors are typically used for sensing pressures. Capacitive sensors can be made highly accurate and repeatable. In gage transmitters that electronically calculate a pressure difference based on two absolute pressure sensor outputs, accuracy and repeatability of the sensors are particularly important to avoid introducing errors in the subtraction process. The barometric pressure range is quite limited, typically 0.9-1.1 atmospheres, and there is a desire to use a relatively low cost absolute sensor for sensing barometric pressure. Low cost absolute pressure sensors, however, often do not have the accuracy and repeatability found in process fluid sensors. These low cost sensors can introduce undesired errors into the electronic subtraction process.
A barometric pressure sensor is needed that can be manufactured at a low cost and that has high repeatability in the limited barometric pressure range.
Disclosed is a pressure sensor comprising a base layer, a sensor layer and a reference layer. The base layer surrounds a passageway between an inlet adapted to receive a pressure and a mounting face on the base layer.
The sensor layer has a first face bonded by an insulating bond to the mounting face. The sensor layer includes a conductive diaphragm aligned with the passageway.
The reference layer mounts on the sensor layer to form a reference vacuum cavity that is aligned with the conductive diaphragm. The reference layer includes a conducting surface facing the conductive diaphragm across the reference vacuum cavity to form a pressure sensing capacitor.